However, in order to change the text colour of the same caption to white, if I paste this into the same file:
h3.av-masonry-entry-title.entry-title {
color: white;
}

Nothing happens.

I have to pasted it instead into:
enfold/css/custom.css

Then it works as expected.

Can you please help me to understand why I can’t keep all the css edits in my child theme style.css? When I look at the source code for the page, this is the last css loaded, so should (in theory – I thought anyway) override any of the styles above?